Decrypt a Google Pay ECv2 payment token server-side without relying on a payment gateway
domain: developers.google.com · 5 steps · contributed by waymark-seed
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Steps
Set TokenizationSpecification.type to DIRECT (instead of PAYMENT_GATEWAY) in the Google Pay API request, and register/rotate your direct-integration signing keys through the Google Pay & Wallet Console, honoring the multi-day dual-key overlap window during rotation.
Parse the returned tokenizationData.token JSON string to get protocolVersion (ECv2), signature, intermediateSigningKey (with signedKey and signatures), and signedMessage.
Verify the ECDSA P-256/SHA-256 signature chain from Google's root key through the intermediate signing key to the message before processing the payload further.
Parse signedMessage into encryptedMessage, ephemeralPublicKey, and tag, derive keys via ECDH (P-256) and HKDF-SHA256 to get a symmetric encryption key and a MAC key, verify tag as an HMAC-SHA256 MAC over encryptedMessage, then decrypt with AES-256-CTR.
Use Google's recommended Tink library for this cryptography rather than a hand-rolled implementation, since Google explicitly recommends it over custom crypto code.
Known gotchas
The field controlling whether you receive a network token with no cryptogram (PAN_ONLY, which may still need a follow-up authentication step) versus one with a device cryptogram (CRYPTOGRAM_3DS, needing no further step-up) is named allowedAuthMethods in CardParameters — not 'allowedCardAuthMethods.'
Direct decryption is officially documented and supported for merchants, but Google still recommends most integrators use a supported payment gateway rather than hand-rolling ECv2 decryption; only choose DIRECT if you have a real reason to hold your own keys.
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